I know I'm replying to an old thread...but just an update for early August for those that may be interested.
We stopped by Tuolumne Meadows on Friday August 3rd at 11am and were told there were only 9 available campsites that morning. We were told the office opens at 8am, and the line for the daily 'first come' sites started forming at 6am.
We stopped by the campground office on August 4th to fill up our hydration packs, and I had a short conversation with a nice woman filling her bottle. She offered to let us stay in her campsite overnight (We ended up near Lee Vining, as everything was full in the Park) to get in line first thing the am. She let us know that she got in line at 5am that morning to get a spot, and there were 9 people already in line in front of her.
Do as Dan recommends, and go with reservations.